The present invention relates to a dielectric ceramic composition adapted for use in microwave applications which comprises three main components of BaO, TiO.sub.2 and WO.sub.3. More particularly, this invention relates to a dielectric ceramic composition especially, but not exclusively, useful for microwave dielectric resonators.
Recently, there have been used dielectric materials having a high dielectric constant, a low dielectric loss and a small temperature coefficient of resonance frequency in resonators. Especially, such materials have been now brought into practical use in applications, such as receivers for a satellite broadcasting, communication devices or equipment, such as automobile telephone, or broadcasting system.
BaO.TiO.sub.2 system dielectric materials have heretofore been known in such applications. For example, dielectric materials comprising Ba.sub.2 Ti.sub.9 O.sub.20 are discussed in detail in U.S. Pat. No. 3,938,064 of H. M. O'Bryan et al, entitled "Devices Using Low Loss Dielectric Material" issued Feb. 10, 1976 (Japanese Patent Publication No. 58-20 905). Further, composite perovskite dielectric materials, typically Ba(Zn.sub.1/3 Ta.sub.2/3)O.sub.3, has been described in Japanese Patent Application Laid-Open No. 53-35 454. However, these known materials have the following disadvantages or problems.
The above Ba.sub.2 Ti.sub.9 O.sub.20 dielectric ceramics are required to be treated with an acidic solution after calcination or to be treated with an oxygen atmosphere over a long period of time after firing, in order to improve their dielectric properties. It has been reported in "Ba.sub.2 Ti.sub.9 O.sub.20 as a Microwave Dielectric Resonator" and "A new BaO.TiO.sub.2 Compound with Temperature-Stable High Permittivity", both presented by H. M. O'Bryan et al at American Ceramic Society 76th Annual Meeting that the dielectric ceramics thus obtained have a Q value of 4200 at 10 GHz, where Q is the reciprocal of dielectric loss tan.delta., i.e., Q=1/tan.delta.. Also, O'Bryan et al further describe in the aforementioned U.S. Pat. No. 3,938,064 that acid leaching of calcined matrials exhibits an advantageous effect of reducing the dielectric loss of the final ceramic products. For example, there are shown improved Q values of 9,400 for HNO.sub.3 leaching, 9,200 for HCl leaching and 9,600 for H.sub.2 SO.sub.4 leaching in contrast to the Q of 4,900 for unleached material. Reoxidation treatment at a temperature of 900.degree. to 1400.degree. C. for a period of 10 to 100 hours is required in certain materials. However, the resulting Q value is insufficient for practical use in resonators and a further increased Q value has been needed.
On the other hand, in the preparation of the perovskite type ceramic material, large amounts of expensive materials, such as Nb.sub.2 O.sub.5 or Ta.sub.2 O.sub.5 are necessitated and will result in an increased cost.
Accordingly, an object of the present invention is to improve the foregoing disadvantages and thereby to provide dielectric ceramics having an increased Q value and a small temperature coefficient of resonance frequency which make them ideal for use in microwave devices. Hereinafter, "temperature coefficient of resonance frequency" is referred to as ".sup..tau. f".